This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improvements in the recovery of seek operations in disc drives having magneto-resistive (MR) heads.
Disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, user data are magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs. A read channel and interface circuit are provided to recover previously stored data from the discs to the host computer.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically used to control the position of the heads relative to tracks on the discs. The tracks are defined from servo data written to the surfaces of the discs during manufacturing. Tracks of equal radial position on adjacent disc recording surfaces are referred to as cylinders.
The servo system carries out two primary operations: seeking and track following. During track following, the selected head is maintained over a selected track on the corresponding disc. To this end, the servo system uses the servo data to generate a position error signal (PES) indicative of the relative distance between the head and the center of the selected track. In turn, the PES is used to generate correction signals which adjust the position of the head by adjusting the amount of current applied to the actuator coil.
During a seek operation, the selected head is moved from an initial track to a destination track on the associated disc surface. A velocity controlled approach is used wherein a velocity profile, indicative of a desired velocity trajectory for the head, is selected based on the distance between the initial and destination tracks. The head is first accelerated to a maximum velocity through the application of a large magnitude of current to the coil. The head then is maintained at this maximum velocity until it reaches a predetermined distance from the destination track, at which point a large amount of current of opposite polarity is applied to the coil to decelerate and settle the head onto the destination track. Adjustments in the velocity of the head are repetitively made during the seek in relation to the difference between the actual velocity and the velocity profile. The desired velocity at any given point during the seek is referred to as a demand velocity, with the velocity profile comprising a series of demand velocity values.
To achieve greater levels of data transfer performance, the heads are typically provided with a magneto-resistive (MR) or similar construction. An MR head incorporates separate write and read elements, with the write element comprising an inductive coil about a core with a write gap and the read element comprising a magneto-resistive material having a changed electrical resistance in the presence of a magnetic fields of a predetermined orientation.
The increased sensitivity of the MR element allows write pulses from the head to be relatively narrow, facilitating higher data transfer rates. However, due to the complexity and minute dimensions of MR heads, it is becoming increasingly difficult to design and manufacture heads that exhibit flawless operation. While manufacturing screening operations attempt to sort flawed heads and remove such from the manufacturing process, some heads nevertheless exhibit intermittent discrepant operation, which is usually manifested as a temporary distortion in the readback signal. Ordinarily, a distorted data readback signal may be corrected with appropriate error correction techniques. However, the servo system, which relies on uncorrected position feedback data from the media, is especially susceptible to this problem. Particularly, if the position feedback signal becomes too distorted during a seek, the servo system cannot determine the position of the head, preventing the head from being correctly settled onto the destination track.
Accordingly, there is a need for improvements in the art to enable disc drives to correct servo signal distortion in an MR head. It is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for compensating for position signal distortion during a disc drive seek.
In accordance with preferred embodiments, a disc drive has first and second data recording surfaces on which tracks are defined, a moveable actuator which supports first and second magneto-resistive heads, and a servo circuit which applies current to an actuator coil to position the heads.
During a seek in which the first head is moved from an initial track to a first destination track, upon loss of transduced servo data from the first data recording surface indicative of radial position of the head, the servo circuit switches from the first head to the second head and applies current to the actuator coil to move the second head to a known radial position with respect to the first destination track while transducing servo data from the second data recording surface. The servo circuit subsequently switches back to the first head and positions the first head over the first destination track in order to carry out desired data transfer operations on the first destination track. By temporarily deselecting the first head, head instability can be resolved by the time that the first head is reselected; in the mean time, the disc drive can continue to operate to bring the first head into alignment with the first destination track.
Preferably, the servo circuit brings the first head to a stop prior to switching to the second head, such as by applying an electrical short to the actuator coil. Moreover, the second head is preferably settled onto a second destination track in the same cylinder as the first destination track so that, when the servo circuit switches back to the first head, the first head is substantially over the first destination track. The second head is preferably the immediately prior head that was utilized before the first head was selected, since this prior head will likely have substantially retained its thermal equilibrium, thus reducing the risk of signal distortion with the second head.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.